In situations where bone fixation occurs near a joint and the surgeon desires to angle the screw to avoid encroachment into the joint, where the surgeon wishes to intentionally angle a screw to cross a fracture gap or joint in order to achieve fusion, or other surgical applications where intentional screw angulation is useful, it is desirable to insert the fixation screw at an angle offset from the central axis of the screw hole in the bone plate and an orientation directed radially about the circumference of the hole. In addition, there are clinical circumstances in which rigidity of fixation is desired, specifically between the screw and the plate, where it is desirable for forces acting on the bone to be counteracted in part or in whole by the screw-plate construct. In these situations, it is desirable to provide a system or mechanism for rigidly locking the screw to the plate where the shaft of the screw can be fixed at various angles to the central axis of the counterpart hole in the plate and at various orientations about the circumference of the screw hole. This type of plate/screw construct is known in orthopedics as a variable-angle locking screw or plate.
Variable angle locking in orthopedic applications is known and utilizes various mechanical designs to secure the head of the bone screw to the plate hole and arranging the shaft and head of the bone screw at variable angles relative to the plate. These designs include plate holes with tapped tabs that cross-thread, self-tapping plate/screw constructs, and frictional “crush-lock” features that provide a degree of rigidity between the plate and screw. However, each of these designs is limited in the strength of the interface between the screw head and the plate due to the reduced surface contact area, and/or reduced mechanical advantage, specific to these features. The tabbed cross-threading and self-tapping designs, for example, include locking features that are a machined as part the plate and thus fixed in position and orientation. To provide a rigid support for a locking screw, the design of the locking mechanism must be optimized for a single screw angle, usually orthogonal to the plate. Deviation from this angle will reduce the surface contact area between the plate and screw therefore reducing the resulting rigidity of the inter-connection. Additionally, placing a screw at the angular limit of the locking mechanism can increase the difficulty of inserting and locking the screw to the plate, further reducing the clinical comfort and confidence of use.
It would be desirable to design, develop and implement a variable angle locking design that enables an orthopedic bone fixation plate to allow a screw to be placed into the bone at various angles, both offset to the plate hole central axis and at radial orientations about the hole circumference, with the same mechanical interface and, therefore, strength, at all angles of screw fixation. This is somewhat achieved with the crush-lock mechanisms found primarily in pedicle screw fixation for spinal applications, however these designs are substantially weaker than the tabbed cross-thread or self-tapping designs when used in plates that are dimensioned for long or small bones (i.e., lacking sufficient depth), if not entirely impractical altogether in that application. The preferred invention disclosed herein improves the strength and rigidity of known variable angle locking plate mechanisms by providing an increased contact area between the screw head threads and the plate regardless of the angle of offset.